1. Field of the Invention
This invention relates to a novel organic compound containing an optically active group and a liquid crystal composition containing the same. More particularly it relates to an organic compound useful as a component of ferroelectric liquid crystal compositions and a ferroelectric liquid crystal composition containing the same.
2. Description of the Prior Art
At present, TN (twisted nematic) type liquid crystal display mode has been most broadly employed as liquid crystal display elements This TN type liquid crystal display mode has many advantages of a low driving voltage, a small power consumption, etc. However, the mode is inferior in the aspect of the response rate to emissive type display elements such as electroluminescence, plasma display, etc. A new TN type display element having the twist angle changed to 180.degree.-270.degree. has also been developed, but it is still inferior in the aspect of the response time. As described above, efforts for various improvements have been made, but they have not yet been realized. However, in the case of a new display mode on which research has recently been earnestly advanced, there is a possibility of notably improving the response rate (Clark et al; Applied Phys. lett., 36, 899 (1980)). This mode is directed to a method utilizing chiral smectic phases such as chiral smectic C phase (hereinafter abbreviated to SC* phase) exhibiting ferroelectricity. Phases exhibiting ferroelectricity are not only SC* phase, but also phases of chiral smectic F, G, H, I, etc. have been known to exhibit ferroelectricity.
Many characteristics are required for practical use of ferroelectric liquid crystal materials in ferroelectric liquid crystal display elements, but at present, there is no single compound which satisfies such requirement. Thus, it is necessary to use a ferroelectric liquid crystal composition obtained by blending some liquid crystal compounds or non-liquid crystal compounds.
Until now, a number of ferroelectric liquid crystal compounds have been reported. Among these, a compound (hereinafter referred to as A) reported in U.S. Pat. No. 4,556,727, ##STR2## exhibits phase transition points of C.40.0.degree. C. (SC*.35.0 .degree. C.) SA.46.8.degree. C.I (C, SA and I being abbreviations of crystalline phase, smectic A phase and isotropic liquid phase, respectively), and its spontaneous polarization value (hereinafter abbreviated to Ps) is also as large as 15 (nC/cm.sup.2); hence it is a very useful compound.
Further, another ferroelectric liquid crystal compound having the same 2-alkoxypropyl group as the optically active group of the compound A has also been reported in Japanese patent application laid-open No. Sho 61-112038/1986.
Still further, there have been reported not only ferroelectric liquid crystal compounds consisting only of ferroelectric liquid crystal compounds,but also ferroelectric liquid crystal compositions as a whole prepared by blending one or more compounds exhibiting ferroelectric liquid crystal phases with a compound or composition exhibiting phases of achiral smectic C, F, G, H, I, etc. (hereinafter abbreviated to phases of SC phase, etc.) as a basic substance (Japanese patent application No. Sho 60-36003/1985). Further, it has also been reported that one or more compounds which are optically active but exhibit no ferroelectric liquid crystal phase are blended with a compound or composition exhibiting phases of SC, etc. as a basic substance to make up the whole into a ferroelectric liquid crystal composition (Mol. Cryst. Liq. Cryst. 89, 327 (1982)).
From the summary of the above facts, it is seen that when one or more optically active compounds, irrespective of whether or not they exhibit ferroelectric liquid crystal phases, are blended with a basic substance, it is possible to constitute a ferroelectric liquid crystal composition. Here, it has been known that the liquid crystalline physical properties of a basic substance have a great influence upon the physical properties of the resulting ferroelectric liquid crystal composition, and also the liquid crystal temperature range, viscosity, etc. of the basic substance have a direct influence upon the liquid crystal temperature range, response time, viscosity, tilt angle, etc. of the resulting ferroelectric liquid crystal composition.
Superior compounds as the above-mentioned basic substance include those containing a pyrimidine ring (Japanese patent application No. Sho 60-132817/1985), those containing a pyridine ring (Japanese patent application No. Sho 61-67961/1986), etc. and it appears that among these, those of directly bonded rings type having no bond group between the rings are particularly superior. Such compounds are also superior in that they have low viscosities and also exhibit phases of SC phase, etc. within a broad temperature range starting from low temperatures.
However, it has been found that when an optically active substance is added to the above-mentioned basic substances of directly bonded rings type to form a ferroelectric liquid crystal composition, there are those which are notably reduced in the ferroelectric liquid crystal temperature range by optically active substance. The optically active substance is preferred to be liquid crystals or substances having a constitution similar to those of liquid crystals, so to speak quasi liquid crystals, but it has been found that even if the optically active substance is liquid crystals or quasi liquid crystals, there are substances which notably reduce the ferroelectric liquid crystal temperature range.
As will be described later in the Reference example, it has been found that when the compounds described above in U.S. Pat. No. 4,556,727 and Japanese patent application laid-open No. Sho 61-112038/1986 are added to a basic substance containing a pyridine ring or pyrimidine ring of directly bonded rings type, the ferroelectric liquid crystal temperature range is notably reduced or extinct and the Ps of the resulting composition is also far smaller than that expected from the addition proportion.